Known cages for electrostatic deposition of powder present a large number of disadvantages. These cages taking in air from outside necessitate a very large external filtering system, with expulsion of the air into the surrounding premises and/or to the exterior. The filters generally comprise a system for reconditioning filtering elements which causes the powder to fall into a tank for recovery of powder. A transporter decants this powder into an auxiliary tank for supplying elements for depositing powder on one face of the piece after having passed through a sifter. One part of this powder may be decanted from the auxiliary tank into another tank by a transport system for supplying projection elements used for depositing powder on the other face of the piece if need be.
The unused powder comes from an external container via a transport system to the sifter. The transport systems are generally pneumatic; it is therefore necessary to add air-powder separation elements of the cyclone type. The coupling tubings take up space and are the site of rapid wear and make access to the various elements of the cage difficult. Generally, these cages do not allow recycling of the air and they also necessitate the conditioning of a significant volume of air coming from the workshop; to do this, a large battery of heating elements in continuous use and a humidifying system are required. A significant improvement can be made by reintroducing the aspiration air, after filtering, into a building containing the projection cage; this means makes the installation complicated and creates problems, more especially because the environment in this building must be acceptable to those who have to work there. Certain cages necessitate someone entering the powdering enclosure to change the type of powder projector; operating conditions are consequently difficult in view of the atmosphere prevailing in the cage and dangerous because of the proximity of heating elements. When completely equipped (tanks, filters, coupling tubing . . . ), these cages occupy a large area and volume in the workshop.